Method and apparatus for producing a measured value indicative of focusing correctly adjusted sharpness at projected pictures

ABSTRACT

The present invention relates to a method and apparatus for carrying out the focusing of projected images. To carry out such focusing, an image sensor operates on a small section of the projected image to blow it up to a point where only the grains of the image become larger than the resolution of the sensor. The image set obtained is electronically scanned so that a signal is received which includes a low frequency band determined by the grain density and a harmonic span generated by the transitions between grains and interstices, whereby a differential voltage is created which is indicative of the state of focusing.

BACKGROUND OF THE INVENTION

Stationary mounted film projectors in cinemas are always located at apredetermined distance from the projection screen in the theater.Focusing the projected image, i.e. adjsuting the optics of theprojector, should, in theory, be a non-recurring task, but experiencehas shown that in practice re-focusing has to be carried out from timeto time. For example, deposits from the surface of the film can build upin the film path of the projector which results directly in blurredimages, and furthermore the layer structure of the film can vary.

The development within the cinema business has lead to that an operatornowadays has to supervise 8-10 projectors in different auditoria. Quitenaturally he is not able then to pay much time and attention to thefocusing of the various projectors, which could be inconvenient to theaudience, since sharpness is a determining factor when it comes toimpression and experience of the picture shown. Also, it can bedifficult to find swiftly the correct setting of focus and, in addition,in certain picture sequences it can be hard to judge the propersharpness of the picture from the projection room. It is often so thatthe projectionist focuses with the guidance of the translated subtitlesoccurring in foreign film along the lower edge of the picture. Thismeans that sharpness becomes poorer where best needed, namely, withinthat part of the screen where the actor's faces mostly appear, i.e. thecentral area of the screen, at a distance from the top edge of thescreen corresponding to about one third of the height of the screen.

In practice there is thus a need for simple and correct focusing means,and earlier several solutions to the problem of focusing have beneproposed. For example, in the Swiss Patent Specification No. 484 443 anapparatus for focusing projected images is described. The basis of thissolution is the fact that a sharp image exhibits greater contrastsbetween light and dark portions, and that a blurred image includes greytones, i.e. the contrast effect is eliminated. However, it is difficultin practice, in particular when cinematographic pictures are concerned,to utilize grey tone conditions for providing focusing parameters.Several specific steps have to be taken in order that normalfluctuations in image intensity be eliminated and in order that the twomeasuring signals be obtained which are to form the basis of acorrecting signal.

SUMMARY OF THE INVENTION

In black and white film the colloidal silver is collected in tinygrains, and in color film the color substances form in a correspondingmanner flocks, whereby in both cases the film obtains a granulatedsurface. In correct focusing this granularity can be distinctly seen inthe projected image, although not from normal viewing distance. Thepresent invention utilizes the occurrence of the image grains, or thelike discontinuities, and generally it is proposed that a section of theprojected image be scanned by means of an image sensor with respect toindividual image elements, that is, projected grain structures, thesignal obtained by the image sensor being fed in the form of a frequencyspectrum to means for separating one high and one low frequency band,which in turn are treated in order to provide a measurement valueindicative of focusing.

In order to transfer the grain structure of the projected image, ascanning image sensor such as a video camera or the like is preferablyused, which delivers an electrical signal related to the grain structureand including a frequency spectrum. The size of the test section, whichis suitably rectangular, is adjusted such that the granularity fallswithin the resolving capacity of the image sensor. Thus the signalobtained by the scanning will contain a frequency spectrum related tothe grain density. The spectrum contains higher and lower frequencies,and according to the invention one higher and one lower frequency bandare selected, a voltage being created on the basis of these bands bye.g. comparison, which voltage is indicative of focusing. The lowerfrequency band will then be related to grain density and the higherfrequency band to grain contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawing illustrating an embodiment of the invention.

FIG. 1 is a diagrammatical view of a cinema plant to which the presentinvention is applied.

FIG. 2 is a view, likewise diagrammatical, of the grain structures in asection of a projected image.

FIG. 3 illustrates a curve obtained when scanning a grain structureaccording to FIG. 2.

FIG. 4 illustrates the grain structure appearance when the image is outof focus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The arrangement according to FIG. 1 shows a stand 1 carrying a lightprojecting device 2 and an associated projector 3. The projector 3 is inturn associated with a projection lens 4 having a rotatable focusingmember 5. The luminous beam 6 from the projection lens 4 is directedtowards a projection screen 7.

Adjacent said projection screen 7 a scanning image sensor 8 is provided,which is adapted to take in by its lens 9 a small section 10 of thepicture projected on the screen. The signal output of the image sensor 8is connected, by way of a line 11, to a threshold circuit 12 whichcommunicates via a line 13 with a filter circuit 14. The filter circuit14 is adapted to select two frequency bands, one lower frequency bandand one containing the harmonics of the lower frequency band. This lowerband, hereinafter designated the grain frequency band, is fed to anoutput 15 and the harmonics frequency band is fed to an output 16.Rectifiers 17 and 18, respectively, are connected to said outputs, therectifiers feeding each its input 19 and 20, respectively, to acomparison circuit 21 in order to provide a resulting voltage output ona line 22 to an amplifier 23 in turn feeding an indicating instrument24. Via a connection 25 the amplifier 23 also feeds a servo amplifier26, which controls in turn a servo motor 27, communicating by way of ashaft 28 or the like with the focusing member 5 of the projection lens4.

The image caught by the image sensor 8 consists of a greatly enlargedportion of section 10. The enlargement should be such that the grainsoccurring on the image surface of the sensor 8 are larger than theresolution of the image sensor 8. In FIG. 2 two grains 29, 30 are shown,representing a sharply defined image. If the path of the scanningelectron beam during a sweep along line 31 in the said figure isfollowed, it can be established directly that after passing the grain 29the beam will run a certain distance 32 until the edge of the next grainis reached. The section between the grains is bright, and a sweep willresult in a voltage variation related to the dark grains and the brightportions therebetween. Thus when an electron beam sweeps across aplurality of grains, the grain frequency band is obtained, which isrelated to the spacing of the grains. As a matter of course, in practicethe interstices between dark grains could accommodate lighter grains, orgrains of varying opacity. However, the frequency bands will be formedequivalently.

In transition from a dark portion, i.e. a grain as shown, to the spacebetween the grains, a jumping voltage change appears in the outputsignal and likewise there is a jumping change when the electron beamreaches the next edge of the grain. The curve shown in FIG. 3illustrates this procedure, the curve line 33 showing the voltage levelwhen the beam passes the grain 29, whereas curve line 34 shows saidlevel when the beam passes the interspace, that is the bright field 32.At the transition a voltage swing occurs, that is, a transient voltage35 forming harmonics to the grain frequency band previously mentioned.Likewise there is a voltage reduction at the transition from the brightportion 32 to the grain 30, said reduction also causing a voltage swing36, later passing over into the voltage level 37, representative to thedark grain 30.

The circumstances now described relate to a sharply focused image, i.e.where the edges of the grains are completely defined, for which reasonthe transitions between light and dark portions become very distinct.FIG. 4 illustrates a sweep 38 across two grains 39 and 40 representing asituation when the projected image is out of focus. As can be seen, thegrain boundaries are no longer distinct and sharp but blurred asindicated in the figure, said boundaries being designated 41 and 42,respectively. When the beam sweeps across e.g. the grain 39 andapproaches the edge to be left, the beam will pass a zone 43representing a light intensity which gradually changes from theblackness of the grain to the bright interspace. Likewise there is asimilar area 44 where the electron beam enters its path across the grain40. The remaining interspace 45 is less than the white field 32 of FIG.2. The diffuse transition sections at the grain edges cause thetransient voltages to be considerably lower in relationship to thoseobtained at sharp focusing.

The arrangement of FIG. 1 operates in the following way. As shown it isassumed that a section 10 is picked up by the image sensor 8, saidsection being located at a distance from the top edge of the projectedpicture corresponding to about one third of the height of the picture 7.As mentioned by way of introduction, it is within this area the greatestsharpness is necessary in order to offer the spectator a pleasant viewof the picture per se. The output signals from the image sensor when thepicture is in sharp focus have, for example, the shape illustrated byFIG. 3. In order that distinct signals for processing be attained, thesignals arriving from the image sensor by line 11 are passed through athreshold circuit 12 which is adjusted taking practical conditions intoconsideration, e.g. basic noise signals. The outgoing signals in line13, thus containing a frequency spectrum, are now treated in the filtercircuit 14 to separate the grain frequency band, which is delivered atoutput 15. The frequency band including said harmonics, generated by thetransient oscillations 35, 36 are delivered to output 16. The separatedfrequency bands containing the signals are rectified individually by therectifiers 17 and 18 and are fed to the respective inputs 19 and 20 ofthe comparator 21. As a a result there is obtained from the latter adifference signal in line 22. This signal is applied to the amplifier23, the strength of the signal obtained being possible to read on theinstrument 24 connected to said amplifier. As a matter of course it isalso possible here, by reading instrument 24, to focus manually beoperating the member 5 of the projection lens 4. Thus maximum instrumentdeflection indicates best focusing. However, the signal obtained herecan control, by way of the connection 25, the servo amplifier 26operating the servo motor 27 in order to focus automatically byactuating said member 5 via the shaft 28. The servo amplifier 26 must ofcourse include such compensation or feedback circuitry which controlsfocusing so that servo hunting is avoided.

If the projection screen 7 should show a blurred image as previouslyexplained a grain representation as shown in FIG. 4 would occur. Theharmonics generated hereby will exhibit a lower amplitude in relation tothe case when sharply focused, with the result that the voltageappearing at the output 16 of circuit 14 will reach a relatively lowerlevel. This, in turn, will have as a consequence that the resultantvoltage, appearing in line 22 for the controlling amplifier 23 andinstrument 24, will have a lower value. By the feature that the grainfrequency band and the harmonics frequency band at all time are relatedto one another by the circuit 21, errors in measurement are avoided. Thelight intensity affects the amplitude of both frequency bands and bytreatment in the comparator circuit an action of relating is broughtabout with the result that only one output voltage is obtained, which isconditioned by differences. The comparatively lower voltage occurringwhen an image is out of focus appears in lines 22, 25, with the resultthat member 5 is operated such, by the servo motor 27, that thecondition of sharpness is restored.

The advantage of using image grains, and distribution of image grains,is obvious as here it is not necessary to consider grey tones previouslydealt with. The grey tones themselves are, as a matter of fact, theresult of grain density. By measuring on grains and interspaces, orlight and dark grains, one has at all times an unambiguous startingpoint, namely, the disposal of distinctly dark and distinctly brightportions as parameters in the measurement procedure.

In connection with projecting cinematographic film there are appearing,on the one hand, frame changing intervals and, on the other,intermediate intervals of shuttering causing dark periods. Hereby alight changing frequency arises which seemingly could disturb thefunction of the arrangement disclosed above. However, said frequency isso low that it can be excluded by filters from the other frequenciesoccurring in the arrangement. But to avoid in certain cases disturbancescaused by said frame changing (and possibly so called image swing causedby deficiencies in the film feeding mechanism, film shrinking and thelike) scanning of the section 10 of the picture area 7 can take placeduring short intervals in synchronism with the frame feed itself. By theuse of modern technics, however, the scanning processes of any screensection can be conducted during very short intervals and, as a matter offact, during each moment of projection the picture frame itself isstanding still in the projector.

In this connection it should be pointed out that the servo device 26, 27could be controlled in such a way that it is activated intermittentlyduring the course of the film show, that is, it is not in operationcontinuously, which could be an advantage during certain conditions ofprojection.

The invention can also be used for checking the quality of projectionequipment. Together with a reference film having a known image sharpnessa measure of the quality of the equipment can readily be obtained byreading the deflection of the instrument 24 and comparing thisdeflection with that obtained when using standardized equipment. In sucha case the apparatus according to the invention should preferably bemade portable.

An important application of the invention is related to the qualitycontrol of a movie film print. In many cases the price of a film printis dependent on the quality of the copying work. By running a perfectfilm print in a standard projection plant, a reference value of thequality can be read on the instrument 24. When running other prints thereadings on the instrument are compared with those obtained when thereference print was run. In order to ensure that a correct quality valuebe read, it would be advisable to connect, instead of an instrument, aprinter, possibly preceded by a digital processing circuit.

Raw film, exposed from a suitable focus test chart and developed, canalso be quality controlled in an equivalent manner.

As a matter of course it is possible within the scope of the inventionto apply digital technics in the present connection for counting againfrequency and harmonics, in doing which the information obtained can beprocessed in a micro-computer in turn controlling the focusing of theprojector. Of course other types of image sensors other than videocameras can be used in the present case for attaining the same goal.What is essential is that there is obtained, on the one hand,frequencies conditioned by the grain or the like discontinuityinterstices and, on the other hand, a formation of harmonics relatedthereto at the transitions between dark and light portions.

What is claimed is:
 1. A method for producing a measurement value tocorrect focusing of projected images or indicative of film printqualities or projection apparatus qualities including the followingsteps:scanning a section of the projected image by means of an imagesensor with respect to individual image elements such as grains orsimilar discontinuities; feeding a signal obtained by the image sensorin the form of a frequency spectrum to a means for separating a lowfrequency band and a high frequency band, said low frequency bandcomprising a frequency or those frequencies generated by a sweepscanning of grains or similar discontinuities occurring in a scanningpath and said high frequency band comprising transient oscillationsforming harmonics to the frequency or frequencies of the low frequencyband; wherein generation of said transient oscillations is performed atthe transition from grains to interstices between grains and frominterstices to grains, or at the transition between grains of differentopacity; converting said low and high frequency bands to signals orstates which are subjected to treatment for comparison so that aresultant value is obtained representing a difference between thesignals or states, said value being indicative of focusing or indicativeof film print qualities or indicative of projection apparatus qualitiesindependent of the light intensity of the image.
 2. The method accordingto claim 1, wherein reproducing of the image section on the image sensoris selected such that the grain size becomes greater than the resolutionof the image sensor.
 3. The method according to claim 1, wherein saidresultant value is read when running a reference film in standardizedequipment, the value read being compared with those values obtained whenrunning the reference film in equipment to be controlled with respect toits quality.
 4. The method according to claim 1, wherein a referencefilm is run in standardized equipment, said resultant value being readduring such running, after which those values obtained when runningfilms to be quality controlled are compared with said first read values.5. The method according to claim 1, wherein the reference film consistsof a film exposed from a focus test chart and developed, and that thosefilms to be quality controlled are lengths of raw films which have beenexposed and developed in an equivalent manner.
 6. An apparatus forproducing a measurement value to correct focusing of projected images orindicative of film print qualities or projection apparatus qualities,comprising:a scanning image sensor adapted to scan a section of aprojected image, the output of the image sensor being connected to aseparator circuit for separating a low frequency band and a highfrequency band, which are then directed to a comparator circuit incommunication with an indicating means which is indicative of focusingoptimal sharpness of the projected image.
 7. The method according toclaim 6, wherein signals which form said low and high frequency bandsare rectified in rectifiers introduced between said separator circuitand said comparator circuit.
 8. The method according to claim 6, whereinsaid indicating means comprises an amplifier and an indicator instrumentconnected to the amplifier.
 9. The method according to claim 8, whereinservo means is connected to said amplifier and controlled by the outputsignals therefrom, said servo means being connected to members forfocusing in projection equipment the image projected.
 10. The methodaccording to claim 6, wherein the output of the image sensor is incommunication with the separator circuit by way of a threshold circuit.11. The method according to claim 9, wherein means for controlling thefunction of the apparatus is brought into operation only intermittentlywhen the projected image appears, whereby the periods of operationbecome synchronized with the frame frequency of the projectionequipment.